Most remote imaging techniques can be broadly divided into two categories: passive and active. As used herein, "imaging" is defined as creating a representation of a scene. Passive imaging techniques detect light emitted or reflected by objects. Passive imaging techniques can be used to recognize objects if the reflective or emissive signature of the objects can be distinguished from sensor noise and background light. As used herein, an object's "signature" is defined as the appearance of the object. A camera is an example of a device which uses a passive imaging technique.
Active imaging techniques direct light onto a target scene and detect light reflected back from objects in the target scene. As used herein, "target scene" is defined as the region in space to be imaged. Similar to passive imaging techniques, active imaging techniques can be used to recognize objects within the target scene if the reflective or emissive signature of the objects can be distinguished from sensor noise and background light. The problem of recognizing an object using remote imaging techniques is complicated because the signature of an object varies with surface, lighting, environmental conditions, viewing angle and percent exposure.
Laser radar, also referred to as Light Detection and Ranging (LIDAR) or Laser Detection and Ranging (LADAR), is an active imaging technique which utilizes a laser in a radar system configuration to remotely image a target scene. Laser radar systems utilize principles of optics and microwave radar. Conventional laser radar systems are able to measure the shape, position, and velocity of objects in a target scene. Other known laser radar systems include coaxial passive thermal sensing receivers which measure the temperature of objects in the target scene.
Conventional laser radar systems can be broadly divided into two categories: scanning and scannerless. Typical scanning laser radar systems include a laser, scanning optics, a timing system, a detector system and a processor. The detector system includes a light detector. To image a target scene, a typical scanning laser radar system first transmits a short pulse of light toward a point in the scene. In one known system, the pulse of light has a duration of approximately one nanosecond. Next, the detector system detects light reflected back from the point in the scene and the timing system determines the round-trip travel time of the pulse of light. As used herein, the "roundtrip travel time" of a pulse of light is defined as the amount of time between the time that the laser transmits the pulse of light and the time that the detector system detects the reflected light. Next, the processor records the direction of the output of the laser and the round-trip travel time of the pulse of light. The scanning optics then position the output of the laser toward a new point in the target scene and the laser radar system transmits a second pulse. This process is repeated for each point in the target scene. Finally, the processor generates an image of the scene in response to the recorded directions of the output of the laser and the corresponding round-trip travel times of each of the transmitted pulses of light.
Typical scannerless laser radar systems include a laser, a timing system, a stationary detector system and a processor. The detector system includes an array of light detectors. To image a target scene, a typical scannerless laser radar system directs the output of the laser toward the target scene and the laser transmits a pulse of light toward the target scene which illuminates the entire scene. Next, the detector system detects light reflected back from the scene. The timing system then determines a roundtrip travel time of the pulse of light for each of the light detectors in the array that detects reflected light. Next, the processor records the positions of the light detectors in the array that detected the reflected light and the corresponding round-trip travel times of the pulse of light for each light detector. Finally, the processor determines an image of the target scene in response to the recorded positions of the light detectors and corresponding round-trip travel times of the transmitted pulse of light.
In general, known laser radar systems are subject to one of the following shortcomings: being heavy and large in size; having cryogenic cooling requirements; having high false alarm rates; having poor ranging precision; and not being capable of single photon detection. Known scannerless laser radar systems have the additional shortcoming of crosstalk between the light detectors in the array of light detectors.
What is desired then is a light-weight direct detection laser radar system which can measure and image the three-dimensional spatial structure of objects located in a target scene with extreme sensitivity. The present invention permits such functionality.